The kidney regulates water/salt balance, nitrogenous waste, blood pressure and blood composition. Kidney function is vital to homeostasis of the body fluids. Chronic kidney disease affects one in nine Americans, a similar number are at risk. The heath care, societal and individual toll of kidney disease is profound. A rigorous understanding of normal kidney development and repair should underpin logical efforts to design new therapeutic strategies for this vital organ system. The basic functional unit of the kidney is the nephron. The full complement of nephrons, approximately half a million in man, form over a lengthy period of fetal life. In this process, uncommitted mesenchymal progenitors are induced to form epithelial nephron precursors, the renal vesicle and other cellular components of the adult organ. The renal vesicle undergoes a complex morphogenesis and patterning process to establish the mature nephron with its distinct physiological domains positioned appropriately along the length of the epithelial tubule. Depending on the mammalian species, nephrogenesis is complete in fetal or early post-natal life. Thereafter, the only option is to maintain and repair these early-formed structures. The long-term goal of our research effort is directed at defining the molecular and cellular processes that establish a functional physiological system during the course of organ development. Given the clinical importance of the kidney, its well-defined physiological roles and the rich history of embryological study over several decades, this organ is an attractive model. We will combine genetic and genomic approaches to understand how the kidney is built from its constituent cell parts in the mouse. In Aim 1, we will define the cellular complexity of the developing kidney's progenitor pools, determine the progenitor-product relationships between uncommitted cells in the kidney and their mature descendants, and identify the cellular mechanisms that maintain progenitors thereby establishing the full complement of nephrons. In Aim 2, we will specifically address the transcriptional actions of opposing regulators, Six2 and _- catenin/canonical Wnt signaling, on the maintenance and commitment of purified nephron progenitors. The transcriptional programs of each will be examined by transcriptional profiling and direct DNA-binding studies and the regulatory networks mined from these data sets. In Aim 3, we will determine whether Notch, a proximalizing factor in the context of the epithelial nephron, can directly induce mesenchymal nephron progenitors to form clinically relevant proximal cell fates, notably podocytes. We will utilize similar strategies to those above to understand Notch's regulatory action in the critical early patterning processes that establish functional cellular asymmetry in the developing nephron. Project Narrative Chronic kidney disease affects one in nine Americans, a similar number are at risk. Determining the normal mechanisms of organ assembly and repair are high priorities. The proposed study utilizes mouse genetic models and genomic strategies to determine the molecular and cellular processes that establish a full complement of functional nephrons during mammalian development from pools of early renal progenitor/stem cells.